Running Benchmark AHB2 in bridged mode and 4 Ohm Speaker

George I agree with you that with efficient low impedance speakers the OP may likely be better off with a single AHB2.

However it’s worth noting that unlike most power amps, the Benchmark AHB2 does not have increasing distortion into lower impedance loads. The bridged AHB2’s distortion is just as low as in stereo mode - the THX AAA circuit takes care of that - the quoted THD+N spec is the same for stereo/mono "at full rated output into any rated load". This has been corroborated by independent measurements. Note that Benchmark only provides power ratings for nominal 8 and 6 ohm loads in bridged mono.

In my case the bridged AHB2 amps do sound better than a single AHB2, but my ATC speakers are very different from the OP. The ATC’s are inefficient with high power handling (ATC recommends 75-300W) and have a nominal 8ohm rating with a minimum of 5.6ohms - pretty much the ideal application for the bridged AHB2.
Using the amps in mono mode also allows for much shorter speaker cables, which may also have contributed to my positive impressions vs a single stereo AHB2.

Having said the above, Benchmark have demoed with bridged AHB2’s on several occasions with nominal 4ohm rated speakers. So clearly they don’t see any issue. Here is a recent positive impression: 
https://parttimeaudiophile.com/2018/05/21/axpona-2018-benchmark-stays-smooth-with-martin-logan/

With all due respect George, it's not a matter of believing what I want.
Implying higher distortion in bridged mode for the AHB2 is incorrect.
This has been independently verified.
Not sure why you keep insisting on this?

https://www.stereophile.com/content/benchmark-media-systems-ahb2-power-amplifier-measurements

I would expect it to be higher. If the rails are 60 volts then the peak volts should approach 120, say 100 Volts with losses, but the current won't support that as 25 amps are needed.

The manual states:
Output voltage: >80Vpp into any load
Output current: 29A peak into 1 ohm, both channels driven

 BTW 18 amps is not an impressive amount of current for modern amp of that size. Should be more like 40 amps. 

I presume you mean current compared to power output not physical size?
The AHB2 is very small and lightweight. IMO the power/current output  - not to mention almost non-existent distortion, super high SNR, efficiency, cool running and (most importantly) resulting purity of sound - are very impressive.
Other than bragging rights, I doubt 40A of current has any advantage in driving practical loudspeaker loads.

I would lay money on it, that if your speakers didn’t need those extra watts, you’d be saying one stereo sounds better that two bridged, regardless of the cost saving.

Perhaps.
But you did say that "distortion goes up" with bridged amps - however this is not the case with the AHB2 in bridged mono, nor into lower impedances in stereo mode for that matter. So in that respect it is a bit different to most conventional power amps.

If we put aside the term "damping" , because thats not what really matters, we need to inform people that an amplifier with high output impedance may cause a marked change in the frequency reponse of their speaker. Thats what really matters.

Yes add that to the other cons, the amp starts to behave like a tone control.
(ie: not flat from 20hz to 20khz)

Oh my gosh, another straw man argument.
This may be an issue with some amplifiers. The AHB2 has sufficiently low output impedance that this is not a problem.

They are very similar yes? what do you think will happen if another measurement was done at 4ohm in the bridged mode????? It will be far worse than the non bridged into 4ohms or 8ohms, that’s why they didn’t give the graph even though they probably did measure it.

George, this is speculative at best.
The AHB2 has very comprehensive protection. I suspect the bridged AHB2 will maintain its ultra low distortion into low impedances until protection kicks in.

If you recall I said this in my initial response above:

George I agree with you that with efficient low impedance speakers the OP may likely be better off with a single AHB2.

What I took exception to was your incorrect blanket assumption that bridging the AHB2 would result in higher distortion. This is NOT true for the specified 6 and 8 ohm loads.

Despite what is specified, Benchmark demos at shows with 4ohm nominal speakers (see link above). Do you think they would do this if the sound quality was compromised?

I am just amazed at how well Benchmark marketing has pulled off selling 2x amplifiers.

Why are you amazed Roger?
Some of us own low efficiency 8 ohm nominal speakers - for which the bridged mono AHB2’s are perfect.
Honestly, Benchmark appears to be one of the least marketing oriented companies around. They don’t push BS, they comprehensively measure everything they make and are very much about accuracy and low coloration - not boutique sound tailoring.

None of what gearbuilder's copy/pasted post spiel from another thread/member.  https://forum.audiogon.com/discussions/benchmark-ahb2-amplifier-what-to-expect Has anything to do with the OP's 4ohm speakers, and bridging this amp into them. Sure it's a great stereo amp.

Really?
Lets look at that.
@gearbuilder said:

3) The THD is virtually identical when comparing stereo mode to bridged mono mode. This is achieved through the use of the feed-forward error correction. Every other power amplifier will show a substantial increase in distortion when bridged.

5) The THD produced by the AHB2 does not increase as the impedance decreases. The THD into 2 Ohms, 4 Ohms, 8 Ohms and no load are virtually identical. Again this is a unique characteristic that is achieved through the use of feed-forward error correction.

6) The AHB2 is not just stable into low-impedance loads, it stays clean when driving low impedances. The THD does not change with loading.

Bottom line, the AHB2 is well suited for bridged mono operation into 4-Ohm nominal impedances and the performance is virtually identical to stereo mode except that the power is nearly 4 X higher. Dips in the speaker impedance curve are not a problem and the AHB2 drives these cleanly.

We almost always demonstrate the AHB2 in bridged mono mode at trade shows and have done so with many different hi-fi and pro loudspeakers. In most cases, these demonstrations have used speakers with a 4-Ohm nominal input impedance.

Could the OP get more than sufficient volume with a single AHB2? Most likely.

But @geek101 asked this:

Does running this amp in bridge mode mean each channel will see half the impedance i.e 2 Ohm each when connected to a 4 Ohm speaker. If so will this cause a problem when the speaker dips to 3 or 2 ohms?. 

The answer to the first part is 'yes' the amp will see half the impedance in bridged mode.
The answer to the second part is "the AHB2 is well suited for bridged mono operation into 4-Ohm nominal impedances and the performance is virtually identical to stereo mode except that the power is nearly 4 X higher. Dips in the speaker impedance curve are not a problem and the AHB2 drives these cleanly.".

What's the problem?

ramtubes said:

I was surprised to see that the ATC 19 is actually 19 liters which is only 0.67 cu ft. I can’t imagine why someone wants to connect this little guy to a 370 watt bridged amplifier. The speaker only goes down to 55 Hz. The resonance can be seen on the impedance graph.

It’s a sealed cabinet design and bass rolls off slowly providing pleasing extension into the mid 40’s in room.
Obviously 380W is not required (or used by me) but 150-200W is useful with the low sensitivity SCM19’s if one wishes to explore the considerable max spl of which they are capable (108dB). ATC recommends 75-300W, though to be fair a single AHB2 drives the SCM19’s very well.
Since my equipment resides in a room corner I also wanted to exploit the placement flexibility of mono blocks which allows positioning behind the speakers and short speaker cables (which should also mitigate loss in damping factor from bridging - not that DF is an issue ;-)
In regard to your other snipe at me, go back and read the first line of my first response in this thread. My subsequent posts were then attempts to correct (apparently deliberate) misinformation. Something you apparently have no problem with.

@ramtubes said:

Im not interested in getting into a fracus with anyone on here however you have stood fast and been a little in everyones face about your Bridged Benchmarks. You did not answer my questions about your listening level or power level. How do you know you need those watts?

Roger, with a calm and unbiased mind go back and re-read my posts above. My only interest was in correcting misinformation, which I believe I did in a polite manner - at least up to the 'Straw man' comment, where I admit I lost patience with the obtuse replies. Sometimes it's hard to gauge the mood of a poster. You have completely mis-read me ;-)

There is no evidence that "It’s a sealed cabinet design and bass rolls off slowly providing pleasing extension into the mid 40’s in room" Granted it is sealed, and thus rolls off at 12 dB/octave.
.................If the speaker is already down 6dB at 54 Hz. (which agrees with the impedance curve) it is going to be down another 6 db or so at 40.

I debated replying to this, since of been accused of excessive self interest - but I like to correct misinformation.
Here is a quote from SCM19 lab report George referenced above:
"Based on this figure, SCM19's bandwidth ranges between 40Hz and 23.2kHz (using the standard -6dB cut-off points) and between 52Hz and 21.6kHz for the standard -3dB cut-off points"
If you're interested in reading the whole review, here is the link:
http://www.avmentor.net/reviews/2016/atc_scm19_2.shtml

Im an information junkie, Ive been correcting things here the whole time while you keep fussing with George about whether this tiny distortion changes or not. No one is going to hear this low level of distortion! 

I agree that the AHB2's distortion is so low it matters little whether it doubles or not into low impedances (it doesn't at any rated load). 
However the inference by George - and this is important to the OP's question - is that the AHB2 will sound worse when bridged. This may be true with other amps but is NOT true with the AHB2.
George goes on to infer stability issues which are also baseless.

THE OP JUST WANTED TO KNOW IF HIS IMPEDANCE DIP WAS GONNA BE A PROBLEM. Many of us advised him it would and further he might get more power with one amplifier in stereo and a lot safer.

What do you base this on? Experience with other completely different designs?
Benchmark's John Siau says it won't be an issue - but what would he know right?
FWIW - and I've repeated this many times - my first line in response to this thread was "George I agree with you that with efficient low impedance speakers the OP may likely be better off with a single AHB2".

Besides the "strawman" snipe, you haven’t been very nice to George. He is in the industry.While he and I don’t agree on everything, we disagree on many things, yet our public conversation here has remained civil and I thank him for that.

As I said above I lost my patience with the 'straw man' comment. Otherwise I've been polite and careful about what I wrote. Read my comments again without bias.

I am certain ATC makes a very fine speaker. I think their maximum recommended amplifier playing heavy music would but an end to the speaker. The powered version only has 35 watts on the tweeter, you have 350 watts which is an accident waiting to happen. Voice coils do melt and you have no protection against that at all. One full level HF or ultrasonic event and bye bye tweeters.

Not sure why you are taking me to task with this, it has nothing to do with the original thread. I don't think you know much about ATC and their drivers. The tweeter comment is obvious, but what's the relevance?
Well designed, stable, high power amplifiers are only a risk if they are used injudiciously.  

The Stereophile graph referenced above is done at 2.83V which is equivalent to 1W/8ohms and 2W/4ohms. 
Obviously, whether bridged or in stereo mode it won't be effected by any insinuated current limitation.
What will effect the simulated speaker load is higher output impedance - which is doubled with the bridged amp - but because its already usefully low this won't make a substantial (read audible) difference.
It certainly won't turn the bridged AHB2 into a tone control - which is a ridiculous exaggeration - especially in comparison to any tube amp or many other SS amps for that matter. 

The misinformation in this thread from an individual who should know better is a little disturbing. The motive escapes me, but it appears to be deliberately malevolent.

From John Atkinson’s measurement comments:
"Commendably, the frequency response was not affected by the setting of the sensitivity switch, or by using the amplifier in bridged-mono mode. However, the ultrasonic response in bridged-mono mode shelved down faster than in stereo mode, and as the two channels’ output stages are in series in this mode, the output impedance, again including the 6’ of speaker cable, was higher than in stereo, at 0.14 ohm at 1kHz."

And his final comment:
"Benchmark Media Systems’ AHB2 is an extraordinary amplifier. Not only does its performance lie at the limits of what is possible for me to reliably test, it packs high power into a very small package, especially when used in bridged-mono mode. It is truly a high-resolution amplifier."
Read more at https://www.stereophile.com/content/benchmark-media-systems-ahb2-power-amplifier-measurements#LdQ7lf...

I suggest people interested in the AHB2 seek out out tests/opinions from reputable sources that have actual hands on experience with the amp. Technical queries should then be directed to Benchmark themselves to avoid speculation and misinformation.

ramtubes319 posts12-07-2018 4:24am
"The 29 amps will only be supplied by the amplifier if the load resistance is low and the voltage is high enough to produce that current. Perhaps its the order of things. Voltage first, then resistance then figure the current. I think we have all come to agree that bridging does not allow the amplifier to provide more than 29 amps. Bridging just allows twice the voltage which increases the OPPORTUNITY for more current to flow, but its still limited to 29 amps."

Agreed, good post.